Given the context `Falsehoods Programmers Believe`, I think it is referring to how one validates a number submitted from a form (but maybe there are devices like this?). So, if a flowers company signs up and wants to list their number as 1-800-Flowers they would get a validation error on many sites.
When I'm interviewing, I'll usually start with something simple with an input that is a phone number and the first 'questionable' input I'll hand them is 1-800-FLOWERS and ask how they are handling it. There are a lot of interesting edge cases with phone numbers. None super tricky , but it makes for an interesting first set of how someone thinks
